This invention relates to a method for pre-processing a picture signal prior to an operational circuit of a picture reproducing machine such as a color scanner and a color facsimile, and more particularly relates to a method for pre-processing a picture signal for converting a desired reproducible density range of an original picture into an operational density range of the operational circuit of the picture reproducing machine.
In a picture reproducing machine such as a color scanner for plate-making, in order to perform a processing of picture signals such as a masking operation, a color correction operation, and so forth in an operational circuit, usually the picture signals obtained by scanning an original picture photoelectrically are logarithmically converted.
The density range of the picture signals is determined in advance depending on a reproducing method of a reproduction picture. For example, when the original picture is reproduced by a halftone reproduction picture, the minimum and the maximum density values correspond to halftone dot area rates of 0% and 100%, respectively, which correspond to the minimum and the maximum values of the gradation scale required to the reproduction picture, and to a highlight point (the lightest point of the density range required to the reproduction picture) having a highlight density D.sub.H and a shadow point (the darkest point of the density range required to the reproduction picture) having a shadow density D.sub.S, respectively.
Although in a usual plate-making the highlight and the shadow points correspond to the halftone dot area rates of 5% and 95%, however, they correspond to the halftone dot area rates of 0% and 100% in the following description.
The highlight density D.sub.H and the shadow density D.sub.S depend on the original pictures, and the difference between the highlight density D.sub.H and the shadow density D.sub.S or the density range depends on the original pictures.
In the picture reproducing machine, before the picture signals are sent to the operational circuit, usually the highlight density D.sub.H and the shadow density D.sub.S of the original picture are pre-processed so that the density range of the original picture may be in a certain voltage range of the operational circuit.
Such a pre-processing is important to the decision of the density range of the reproduction picture. The controls of the highlight and the shadow densities D.sub.H and D.sub.S of the reproduction picture to the halftone dot area rates of 0% and 100% are commonly referred to as a highlight setup and a shadow setup.
The highlight and the shadow setups also indicate the adjustment of the input conditions for the operational circuit, which are different per each original picture, prior to the start of the picture reproducing machine, and thus these setups are referred to as an input setup in the following description.
In FIG. 1 there is shown a digital color scanner including a conventional pre-processor means, wherein a signal is processed in a direction, as shown by arrows. In FIG. 2 there is shown a graph for signal conversions performed in the color scanner shown in FIG. 1.
An original picture 1 illuminated by a light source 2 is scanned photoelectrically by a scanning means (not shown). A light beam 3 through the original picture 1 is picked up by a pickup head 4 to obtain a picture signal a. Then, the picture signal a is sent to a log-converter 5 and is converted logarithmically there to output a picture density signal b which is then fed to a conventional input setup circuit 6.
The input setup circuit 6 adjusts the density range of the picture density signal b and outputs an operational density signal c so that shadow and highlight values b.sub.S and b.sub.H of the picture density signal b, which correspond to the shadow and the highlight densities D.sub.S and D.sub.H of the original picture, may correspond to minimum and maximum values c.sub.o and c.sub.m of the operational density signal c.
The operational density signal c can be fed directly to an operational circuit of an analog color scanner.
In this embodiment, the operational density signal c is input to an analog-digital converter 7, hereinafter referred to as A/D converter, and is converted into a digital operational density code d which is to be sent to an operational recorder 8.
The operational recorder 8 comprises an operational circuit 9 which carrys out a color correction, a masking, and other correction operations required to the color scanner, a digital-analog converter 10 which converts a digital signal into an analog signal, and a recording head 12 which records a reproduction picture onto a recording film 11 by scanning. A contact screen 13 is attached onto the front surface of the recording film 11, as occasion demands.
The adjustment of the input setup circuit 6 is carried out by allowing to coincide the density range of the operational density signal c with the input voltage range of the A/D converter 7 by the standard level adjustment and the span adjustment of the operational density signal c so that the shadow and the highlight values b.sub.S and b.sub.H of the picture density signal b may be coincident with the minimum and the maximum values d.sub.o and d.sub.m predetermined of the operational density code d which is output from the A/D converter 7, as follows.
For example, two different original pictures 1 and 1' to be reproduced have different shadow densities D.sub.S and D.sub.S ' and different highlight densities D.sub.H and D.sub.H ', and hence the shadow values b.sub.S and b.sub.S ' and the highlight values b.sub.H and b.sub.H ' of the picture density signals b are different.
In order to force the different shadow and highlight values b.sub.S, b.sub.S ', b.sub.H and b.sub.H ' to correspond to the minimum and the maximum values d.sub.o and d.sub.m of the operational density code d, the conversion characteristics of the setup circuit 6 are adjusted, as shown by the characteristics curves A and B for the original pictures 1 and 1' in FIG. 2, so that the shadow values b.sub.S and b.sub.S ' and the highlight values b.sub.H and b.sub.H ' may be converted into the minimum and the maximum operational density signals c.sub.o and c.sub.m.
In the setup circuit 6 the standard level adjustment and the span adjustment are carried out by means of a level shift circuit 14 and a gain control circuit 15, respectively. That is, the level shift circuit 14 shifts the level of the operational density signal c so that the shadow values b.sub.S and b.sub.S ' of the signals b may correspond to the minimum value c.sub.o of the operational density signal c, and the gain control circuit 15 adjusts the inclinations .theta..sub.A and .theta..sub.B of the characteristics curves A and B, i.e. the conversion gains, so that the highlight values b.sub.H and b.sub.H ' of the signals b may correspond to the maximum value c.sub.m of the operational density signal c, thereby setting the standard level and the span.
Meanwhile, since the picture signal a to be fed to the log-converter 5 requires a wide dynamic range such as a three decade range, i.e. a range of three figures of decimals or a density value of at least 3.0, a high stability against a direct-current drift, a gain drift, and so forth, is required to the log-converter 5. The same stability as the log-converter 5 is also required to the setup circuit 6.
Further, a good operability is also required to the setup circuit 6, for example, the adjustments can readily be performed according to the different input conditions of the different original pictures. In a conventional setup circuit 6, the operability and the stability are contrary to each other, that is, the one better, the other worse.
On the other hand, in general, each color separation picture signal of red, green or blue is processed in each color channel through a photoelectric converter, a preamplifier, a logarithmic converter, and so forth, and the deviations of the conversion characteristics or the transfer characteristics, and the linearities of the conversion characteristics are usually different per each color channel. Therefore, it is necessary to correct these values of the different original pictures in each color channel in order to be the same value so that these values corrected may directly be input to the operational circuit of the picture reproducing machine.
In order to remove the deviations of the relative characteristics among the color channels and to correct the linearities of the conversion characteristics in each color channel, for example, it is ideal to convert the picture density signal b into a certain function by using a straight line segment function converter. However, such a straight line segment function converter having a complicated construction is unstable. Further, it is difficult to adjust the straight line segment function converter.